1. Field of the Invention
The present invention relates to a MEMS (Microelectromechanical Systems) structure and a fabrication method of the same, and more particularly, to a MEMS structure having a floating portion above a substrate and a fabrication method of the same.
2. Description of the Related Art
MEMS is a technology for realizing mechanical and electronic devices using a semiconductor manufacturing process. An element manufactured using MEMS technology generally has a bendable portion floating above a substrate for mechanical operation.
FIG. 1 is a drawing showing an example of a MEMS structure.
The MEMS structure has a substrate (110), a fixing portion (131) fixed on the substrate (110) and a floating portion (133) extended from the fixing unit (131). The fixing unit (131) is commonly referred to as an anchor or a support and has a function of fixing the floating portion (133) to the substrate (110). The floating portion (133) floats above the substrate (110) thereby bending upward and downward by an external driving force as shown by the two-dotted line. The floating portion (133) can be formed as a beam or membrane if necessary.
FIGS. 2A through 2D are drawings illustrating consecutive examples of the manufacturing process of the MEMS structure.
On the substrate (210), a sacrificial layer (220) is deposited and the sacrificial layer (220) is patterned to have a form of an anchor fixed on the substrate (210) as shown in FIG. 2A. On the patterned sacrificial layer, (220) a MEMS structure layer (230) is deposited as shown in FIG. 2B. As the MEMS structure layer (230) is deposited, a floating portion (233) is formed on the part where the sacrificial layer (220) remains, while a fixing portion (231) and a connecting portion (232), which connects the fixing portion (231) and the floating portion (233), are formed on the part where the sacrificial layer (220) is removed.
Next, etchant access holes (240) are formed on the floating portion (233) of the MEMS structure layer (230) as shown in FIG. 2C. When etchant is applied to the sacrificial layer (220) through the etchant access holes (240), the sacrificial layer (220) is removed and the floating portion (233) becomes separated from the substrate (210) as shown in FIG. 2D.
Depositing the MEMS structure layer (230) on a patterned sacrificial layer (220) and then removing the sacrificial layer (220) is the most generally used method for fabricating a MEMS structure that floats above the substrate (210).
However, the MEMS structure fabricated by this method is problematic since the connecting portion (232), which connects the fixing portion (231) fixed on the substrate (210) and the floating portion (233) disposed above the substrate (210), is relatively thinner than the other parts and therefore very weak. As a result, the connecting portion (232) can be damaged as the movement of the floating portion (233) is repeated. Furthermore, it becomes difficult to have a sophisticated control over the movement range of the floating portion (233) since the connection condition of the connecting portion (232) varies.
FIGS. 3A through 3D are drawings illustrating consecutive examples of the MEMS structure fabrication process using a SOI (Silicon on Insulator) wafer.
As shown in FIG. 3A, a sacrificial layer (320) is deposited on a substrate (310). On top of the sacrificial layer (320), a MEMS structure layer (330) for forming the MEMS structure is deposited. The MEMS structure layer (330) is patterned to have a form of the MEMS structure to be fabricated. When the MEMS structure layer (330) is patterned as shown in FIG. 3B, etchant access holes (340) are also formed. The etchant access holes are formed on a part of the floating portion (333) of the MEMS structure layer (330) but not of the fixing portion (331).
And then, when etchant is applied to the sacrificial layer (320) through the etchant access holes (340), the sacrificial layer (320) is partially removed. At this time, because the etchant access holes (340) are formed only on the part of the floating portion (333), the sacrificial layer (320) under the floating portion (333) is totally removed, and the sacrificial layer (320) under the fixing portion (331) is partially removed along the edges as shown in FIG. 3C.
As the floating MEMS structure is fabricated on the substrate (310) by depositing the MEMS structure layer (330) on the sacrificial layer (320) and etching a part of the sacrificial layer (320), the part of the sacrificial layer (320) that is not removed functions as an anchor fixing the MEMS structure onto the substrate (310).
However, according to this method, precise control of the floating portion's (333) length is difficult because the width of the sacrificial layer (320) that functions as an anchor varies depending on the amount of time the sacrificial layer (320) is exposed to the etchant. That is, if the sacrificial layer's (320) exposure time to the etchant is long, the width of the anchor becomes narrow as shown in FIG. 3D. Conversely, if the exposure time is short, the width becomes wide as shown in FIG. 3C. Accordingly, the length of the floating portion (333) varies. Therefore, this method is not suitable for fabricating a MEMS structure requiring a precise size and length of the floating portion (333).
Additionally, since a MEMS structure fabricated by this method has the sacrificial layer (320) between the fixing portion (331) and the substrate (310), if the sacrificial layer (320) uses an insulating material, it is difficult to electrically connect to other circuits on the substrate (310). Consequently, additional effort is required to connect the MEMS structure to other circuits.